Over the last decade, the optical fibers have been developed and installed as the backbone of interoffice networks for voice, video and data transmission. These are becoming important with growing and expanding telecommunication infrastructure. Their importance is further increasing because of their high bandwidth applicability. The higher bandwidth demand is further increasing exponentially with time because of rapid growth of information technology.
The network capacity optical communication in the world is exploding. The growing bandwidth demand can be met by use of the new generation of dense wavelength division multiplexer, hereinafter referred to as DWDM, approach with low dispersion single mode optical fibers in the 1530–1565 nm (C-band) and in the 1565–1625 nm (L-band) wavelength ranges. The requirements of the fiber have had to change to support these advances, especially the requirement for the higher spot area and the amount and uniformity (slope) of chromatic dispersion across these wavelengths. The DWDM approach enhances the effective data rate of an optical fiber link by increasing the number of wavelength channels within the wavelength band.
Conventionally, the multi-mode fiber at wavelength of 850 nm were used, which were replaced by single mode fibers with zero dispersion wavelength near 1310 nm. The single mode or monomode optical fibers have greater bandwidth than that of the multimode fibers.
Therefore, the research has been directed towards the development of the single mode fibers, as these fibers were observed to have lower attenuation between the wavelength range from 1300 nm to 1550 nm.
However, when single wavelength moved through 1550 nm window for lower attenuation, the single mode fibers were observed to have very high dispersion.
The major disadvantage of the known single mode fibers with high dispersion at 1550 nm was that, it obstructed higher bit rate is transmission. This disadvantage of single mode fibers has been overcome by the improved single mode fibers, known as dispersion shifted fibers, which have zero dispersion even when the wavelength shifted to 1550 nm.
The theoretical analysis reveals that a single mode fiber having Low dispersion and low dispersion slope with higher effective area is most desirable for high capacity DWDM, as referred hereinabove, transmission. However, the dispersion shifted fibers used for long distance systems in the prior art have higher dispersion which promotes poor DWDM performance. The dispersion flattened fiber which specify the dispersion magnitude less than 2 ps/nm.km between 1.3 to 1.6 μm have zero dispersion region within the DWDM window. This result is strong four wave mixing, which prevents good DWDM performance.
Ideally the dispersion of an optical fiber should have a constant value over the entire wavelength-operating region. However, the dispersion of fibers varies with the wavelength as the refractive index varies with the wavelength. Their dispersion slope S0 quantifies this variability. The smaller the slope the lesser the dispersion varies with the wavelength. Another advantage of the low dispersion and low dispersion slope fiber is that its small dispersion allows its minimum dispersion to be increased to better suppress the Four Wave Mixing non-linearity, while still keeping the fiber minimum dispersion small enough for the signals to travel to longer distances with minimum need for dispersion and dispersion slope compensation.
It has been observed that the bandwidth or the capacity of the Lightwave systems can be expanded in different ways. A) Increasing the number of wavelengths within the fiber (DWDM approach). B) Transmitting at a faster speed (Time division multiplexing) or C) By increasing number of fibers within the cable. Power requirements of the optical amplifier limits the more fiber counts within the cable.
This is the fact that systems push the performance to the limit. Hence, the roll of a fiber in the system becomes critical. As stated hereinabove, the fiber characteristic should remain under control, particularly the dispersion must be balanced between the requirement for compensation and the suppression of non-linear effects, the effective area must be larger to reduce the non-linear effects without affecting the fiber performance, the dispersion slope must be low enough to reduce the inter channel spacing i.e., for all channels to propagate with an extremely low errors in bit rate, etc.
The increasing complexity of the demands on the fiber makes the designer to think further to re-optimize the refractive index profile. This requires thinking to have more complex designs. However, the complex designs are very sensitive to the manufacturing processes The optical and material physics limits the combination of the above-said parameters, which can be achieved. The end product is the compromise, where each parameter is optimized to the best value, which can be achieved without adversely affecting performance of the critical attributes and system requirements. Insensitive system modeling is done with each varied parameters to understand its impact.
The parameters, like refractive index and radius of each part of the fibre, like centre core, cladding(s), ring core(s) and outer core, and the relationship between refractive index and radius of such parts of the fibre, and number of cores and claddings decide the characteristic properties of thus obtained fiber and the applications of thus obtained fiber.
Therefore, the fibers known in the art are distinguished by way of their characteristic properties, which in-turn are decided by various parameters as stated herein above. The fibers as known in the prior art either have low non-linearity but high bend loss or have low bend loss but less effective area or may have higher non-linearity and higher bend loss or may have non-uniform chromatic dispersion over the third and fourth window or high dispersion slope, that is the fibre will not have optimum characteristic properties and will sacrifice one of the property for achieving another property.
It has been observed that the dispersion and dispersion slope varies with the wavelength and refractive index varies with the wavelength.
Therefore, in view of variation of dispersion and dispersion slope with the wavelength and variation of refractive index with the wavelength constant efforts are being made to develop optical fibers which have optimum dispersion and dispersion slope and yet having higher spot area and such a refractive index profile and the configuration which is easy to be achieved and accordingly it is easy to fabricate the desired fiber which is suitable in as wider range of the wavelength as possible.
Therefore, the inventors of the present invention have made an attempt to develop the fiber, which will have optimum characteristic properties, that is which will not sacrifice one of the characteristic property to achieve another characteristic property.
Therefore, there is a need to develop a dispersion and effective area optimized fiber, particularly a single mode dispersion optimized fiber having as far as possible optimum low dispersion slope between 1530 to 1565 nm (C-band) and 1565 to 1625 nm (L-band) transmissions along with higher effective area. More particularly, the need is to develop a fiber which is suitable for transmission of higher bandwidth over longer distance with more uniform chromatic dispersion over the third and fourth window and yet has very high effective area and also to achieve low bending induced loss at 1550 nm and at the more critical 1625 nm wavelength.